Graft copolymers are defined as the macromolecule formed when polymer or copolymer chains are chemically attached as side chains to a polymeric backbone. Generally, the side chains are of different composition than the backbone chain. Due to the ability to chemically combine unlike polymeric segments in one molecule, graft copolymers have unique properties, making them useful for their surface active properties, particularly in stabilizing physical blends of otherwise incompatible polymers.
"Macromonomers," as used herein, are defined as polymers or copolymers having relatively low molecular weight, such as an oligomer; having a functional group at the terminal portion of the chain; and are capable of free radical polymerization. Macromonomers have found use in surface coatings, adhesives, plasticizers and surface active agents. The graft copolymer products of the present invention combine the desirable properties of the macromonomers formed in stage one of the polymerization with desirable properties of the polymer or copolymer formed in stage two of the polymerization.
The copolymer products of the present invention can be used directly in water-based emulsion coatings, elastomers, adhesives, caulks and mastics. Still further uses for these copolymer are as plastic additives for use as compatibilizers of polymer-polymer blends.
Macromonomers capable of undergoing free radical polymerization have traditionally been prepared in organic solvents by a two-step process. First an end-functional oligomer is prepared. This is then followed by the conversion of the terminal functionality on the oligomer into a new, free radical-polymerizable functionality. Once these macromonomers have been formed, they can be polymerized with other monomers to yield a graft copolymer, whereby the macromonomer forms side chains on a polymer backbone formed from the other monomers. Similar to the formation of the macromonomer, this graft copolymerization is usually carried out in organic solvents, as exemplified in U.S. Pat. No. 3,390,206.
Milkovich and Schulz have demonstrated (J. of Applied Polymer Science, Vol. 27, 4473-4486 (1982)) that polystyrene macromonomers prepared in an organic solvent, can then be dissolved in backbone comonomer, emulsified and polymerized by a free radical aqueous emulsion polymerization process. The macromonomers can be used with or without its solvent removed.
A key drawback to these graft copolymerizations is that organic solvent is a necessary component and must be removed at some point if a solvent-free product is desired. In addition, in the approach used by Milkovich et al., the macromonomers formed in an organic solvent do not emulsify well and therefore lead to a final aqueous dispersion of polymers in the form of large particles (1-5 microns). This is also undesirable because the large particles settle rapidly and therefore do not form a stable dispersion.
Japanese Patent Application 62-289293 discloses a process for the production of a macromonomer and conversion of the macromonomer into a graft copolymer in the form of an aqueous suspension. These macromonomers are produced by first forming a prepolymer with terminal functional groups, such as carboxyl groups. This prepolymer is formed, in the absence of organic solvent, by the polymerization of a vinyl monomer in the presence of a mercapto-acid type chain transfer agent, such as mercapto acetic acid. The chain transfer agent imparts the carboxyl group functionality onto the terminal portion of the prepolymers. This prepolymer is preferably made by an aqueous suspension process, and then isolated in a step that requires removal of the water. Alternately, the prepolymer can be made by a bulk polymerization process. An emulsion polymerization process is stated to suffer from low chain transfer efficiency of the mercapto-acid chain transfer agent.
The dry prepolymer is then dissolved in vinyl monomer and reacted with a second compound containing both a vinyl group and a functional group reactive with the terminal functional group on the prepolymer. The reaction is inhibited so that the vinyl monomer used as the solvent and the vinyl portion of the second compound do not polymerize. The reaction of the prepolymer with the second compound is a necessary step to impart vinyl functionality onto the macromonomer. The macromonomer/vinyl monomer solution can then be polymerized by a bulk process or by a suspension process. It is stated that an emulsion polymerization process isn't useful because the emulsion stability of the macromonomer/vinyl monomer solution "is not very good."
Japanese Patent Application 63-148202 discloses a method for producing graft copolymers utilizing an aqueous emulsion polymerization process. A macromonomer is dissolved in vinyl monomer, and this solution is emulsified and then polymerized. The macromonomer can be made by any of the two-step procedures known in the prior art, most of which utilize solvent, and then the macromonomer is dissolved in vinyl monomer. Emulsification of the macromonomer/vinyl monomer solution is extremely difficult and requires the use of very specific surfactants or pairs of surfactant, in conjunction with emulsification via a homogenizer or ultrasonic waves. The process is energy intensive and provides aqueous dispersions with average particle size of 480 nm.
The present invention is unlike the process of Japanese Patent Application 63-148202 in several respects. First, the ethylenically unsaturated groups on the terminal portion of the macromonomers formed during stage one of the present invention are directly attached to the macromonomer in a single step. No post-functionalization step is needed. Second, the macromonomer is readily made in the absence of solvent. Third, the macromonomer formed during stage one of the present invention is already in the desired form of a stable, small particle size dispersion and therefore does not require having to be isolated, dissolved in stage-two monomer, and emulsified, to achieve that form. Also, the dispersion of the present invention is prepared utilizing common surfactants, and does not require homogenization. Fourth, the final graft polymers of the present invention are in the form of a small particle size dispersion (100 nm is easily achieved) which is completely stable to settling. The process of Japanese Patent Application 63-148202 still produces relatively large (480 nm) particles, which are somewhat unstable and slowly settle.